Foreign object damage
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- "FOD" redirects here. For the song by Green Day, see Dookie. For the in gay slang, see Friend of Dorothy.
Foreign object damage or foreign object debris (FOD) is a substance, debris or article alien to the vehicle or system which would potentially cause damage.[1] Foreign object damage is any damage attributed to a foreign object that can be expressed in physical or economic terms that may or may not degrade the product's required safety and/or performance characteristics. Typically, FOD is an aviation term used to describe both the damage done to aircraft by foreign objects, and the foreign objects themselves (i.e. any object that has, or is likely to, cause damage.)[2]
"Internal FOD" is used to refer to damage or hazards caused by foreign objects inside the aircraft. For example, "Cockpit FOD" might be used to describe a situation where a clipboard, water bottle, or other item gets loose in the cockpit and jams or restricts the operation of the controls. "Tool FOD" is a serious hazard caused by tools left inside the aircraft after manufacturing or servicing. Tools or other items can get tangled in control cables, jam moving parts, short out electrical connections, or otherwise interfere with safe flight. Aircraft maintenance teams usually have strict tool control procedures including toolbox inventories to make sure all tools have been removed from an aircraft before it is released for flight. Tools used during manufacturing are tagged with a serial number so if they're found they can be traced.
FOD costs the aerospace industry $4 billion USD per year and causes expensive, significant damage every year to aircraft and parts and may cause death and injury to workers, pilots and passengers.
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[edit] Examples
Examples of FOD include:
- Bird strike: when an aeroplane flies into a bird, the impact can cause severe damage to the fuselage or engine, or even directly injure those aboard the aircraft if the bird strikes the aircraft in such a way as to enter the cockpit and/or cabin.
- Rock or other metal parts: Usually occurs when the aircraft is taking off or landing. The intake suction from a jet engine is often powerful enough to suck up loose material lying on the runway, and the winds created by a helicopter or prop-driven aircraft's rotors can send such objects airborne, creating hazards to nearby personnel. One way to counter this is to install a gravelkit, which exist to certain planes like the early B737.
- Hail: can break windshields and damage or stop engines.
- Ice on the wings, propellors, or engine intakes
- Dust or ash clogging the air intakes (as in sandstorms in desert operating conditions or ash clouds in volcanic eruptions). For helicopters, this is also a major problem during a Brownout.
- Tools, bolts, metal shavings, lockwire, etc. mistakenly left behind inside aircraft during the manufacturing process or maintenance.
All aircraft occasionally lose small metal or carbon parts during takeoff and landing. These parts remain on the runway and can cause damage to tyres of other aircraft, hit the fuselage or windshield/canopy, or get sucked up into an engine. Although airport ground crews regularly clean up runways, the crash of Air France Flight 4590 demonstrated that accidents can still occur: in that case, the crash was caused by debris left by a flight that had departed only four minutes earlier.
On aircraft carriers and military (land) airfields, FOD walkdowns are conducted before flight operations begin. A line of crewmen walk shoulder to shoulder along the full length of the flight deck, searching for and removing any foreign objects. The objects removed are often also referred to as "FOD" although they haven't caused any damage. In this context a more appropriate translation of the acronym would be "foreign objects and debris".
One spectacular accident involving FOD happened during the USSR war in Afghanistan when an Il-76 got too close on the taxiway to a MiG and sucked in the smaller planes drogue chute into one of it's engines.
[edit] Jet engine design and FOD
Modern jet engines suffer major damage due to even small birds being sucked into the engine. The FAA (Federal Aviation Administration) requires that all engine types pass a test which includes throwing a fresh chicken (dead, but not frozen) into a running jet engine. The engine does not have to remain functional after the test, but it must not cause significant damage to the rest of the aircraft. Thus, if the bird strike causes it to "throw a blade" (break apart in a way where parts fly off at high speed), doing so must not cause loss of the aircraft. FAA.gov Circular A chicken gun is used to perform experiments on bird strikes.
[edit] Engine design which avoids FOD
Some military aircraft have a unique design to prevent FOD from damaging the engine. The design consisted of an S-shaped bend in the airflow so that air entered the inlet, was bent back towards the front of the plane, and bent back again towards the back before entering the engine. At the back of the first bend a strong spring held a door shut. Any foreign object flying in the intake flew in, hit the door, opened it, flew through, and then exited the aircraft. Thus, only small objects swept up by the air could enter the engine. This design did indeed prevent FOD problems, but the constriction and drag induced by the bending of the airflow reduced the engine's effective power, and thus the design was not repeated. However, many consider it an innovative solution to a challenging engineering problem.
The Russian MiG-29 fighter has a special engine design to prevent injestion of FOD during take-off from rough airfields. The front air intakes could be closed and special inlets on the top of the plane temporarily opened. This would allow enough airflow to the engine for take-off but reduced the chances of the engine sucking up objects from the ground.
Another interesting design to minimize the risk of FOD is the Antonov An-74 which has a very high placement of the engines.
[edit] FOD damage examples
[edit] Bird strike
[edit] B-1A crash
In the late 1970s, the B-1A bomber began production and the military began flight testing to determine its capabilities and limitations. Very shortly after the aircraft was accepted by the military, there was at least one very high profile crash, shortly after takeoff, from an airfield in North Dakota. The cause of the crash was determined to be FOD. Specifically, the aircraft flew into a flock of geese moments after it lifted from the ground. Many geese struck the leading edge of the wings. Unfortunately, the primary, secondary, and backup hydraulic system lines were all positioned within inches of each other in that section of the wing. The bird strikes dented the aircraft skin, which in turn dented the hydraulic lines and caused loss of pressure in all three systems. The aircraft became uncontrollable and crashed.
It was determined that the aircraft design was the major factor in this crash, not the birds. Any military jet should be expected to operate in unimproved conditions and probably incur FOD during normal operation. Thus, the design decisions were faulty in that (a) all three hydraulic systems were too close together, (b) they were very close to the outer skin of the aircraft, and (c) they had no extra shielding on the skin nearest this juncture point. The B-1A was a short lived model. The B-1B incorporated many changes including rerouting, separating, and shielding the hydraulics.
[edit] Nimrod crash
On 17 November 1980 a Hawker Siddeley Nimrod of the Royal Air Force crashed shortly after taking off from RAF Kinloss. It flew through a flock of Canada geese, causing three of its four engines to fail. The pilot and copilot were killed; the pilot was subsequently posthumously awarded an Air Force Cross for his actions in maintaining control of the aircraft and saving the lives of the 18 crew. The remains of 77 birds were found on or near the runway.[3][4]
[edit] Air France Flight 4590
The crash of a Concorde, Air France Flight 4590, at Charles de Gaulle International Airport near Paris on 25 July 2000 was caused by FOD; in this case a piece of titanium debris on the runway which had been part of a thrust reverser which fell from a Continental Airlines McDonnell Douglas DC-10 that had taken off about four minutes earlier. All 100 passengers and nine crew on board the flight, as well as four people on the ground, were killed.
[edit] British Airways Flight 9
On 24 June 1982, British Airways Flight 9 on route to Perth, Australia flew into a volcanic ash cloud over the Indian Ocean. The Boeing 747-200B suffered engine surges in all four engines until they all failed. The passengers and crew could see a phenomenon known as St. Elmo's fire around the plane. Flight 9 dived down until it exited the cloud allowing the airborne ash to clear the engines, which were then restarted. The cockpit windshield was badly pitted by the ash particles but the aircraft landed safely at Jakarta, Indonesia where passengers were disembarked without injury.
[edit] Wildlife and wetlands near airports
Significant problems occur with airports where the grounds were or have become nesting areas for birds. While fences can prevent a moose or deer from wandering onto a runway, birds are more difficult to control. Often airports employ a type of bird scarer that operates on propane to cause a loud enough noise to scare away any birds that might be in the vicinity. Airport managers use any means available (including trained falcons) to reduce bird populations.
On 29 April, 2007, a Thomsonfly Boeing 757 sustained engine damage from a bird strike during takeoff. The damaged engine exhibited repeated compressor stall until being shut down; the plane returned to the airport safely. The entire incident was caught on video (viewable here).
[edit] Technologies capable of detecting FOD
- Thermal imaging cameras
- Lidar (Light Detection and Ranging)
- Radar
- Camera systems [visible band imagery (standard CCTV) and low light cameras]
[edit] Available automatic FOD surveillance & detection system
- iFerret™ , a World First to utilize intelligent Vision from Stratech Systems Limited (Singapore) to detect and classify foreign objects, with the ability to pinpoint the location of the foreign objects. This revolutionary system features self-calibrating cameras, automated scene analysis and configurable scan resolution for different object sizes. Providing continuous runway inspection in-between flights, it facilitates rapid recovery response through accurate assessment and real-time alert for FOD, bird strike condition, and even battle damage surveillance.
[edit] References
- ^ According to the National Aerospace Standard 412, maintained by the National Association of FOD Prevention, Inc.
- ^ NAFPI website
- ^ Aviation Safety Network XV256 accident page retrieved 2008-01-23.
- ^ "RAAF Exchange Pilot Valour Cited in RAF Accident Report", "Newsdesk - Military", Australian Aviation magazine No. 16, September 1982, p45. Aerospace Publications Pty. Ltd., Manly NSW
